1. Field of the Invention
The present invention relates to a master station of a communication system and an access control method, and more particularly, relates to an access control method, which is used in a plurality of communication systems sharing the same channel, for preventing interference between the plurality of communication systems from occurring.
2. Description of the Background Art
Conventionally, as a technique for reducing interference between a plurality of communication systems sharing the same channel, there exists an access control method for reducing the influence of interference signals by transmission power control. For example, there exists Japanese Laid-Open Patent Publication No. 2002-198834 (patent document 1), Japanese Laid-Open Patent Publication No. 2003-37556 (patent document 2), or Japanese Laid-Open Patent Publication No. 2001-53745 (patent document 3).
Patent document 1 discloses a method for attenuating a signal power and an interference power by an attenuator provided in a base station, and compensating a power of a wireless signal inputted to a receiver with a transmission power of a transmitter of a terminal station so that a power level of the wireless signal becomes a reference level.
Also, patent document 2 discloses a method by which a base station, which has detected interference signals, notifies interference information to another base station transmitting the interference signals via a local communication network for causing the notified base station to reduce a transmission power based on the interference information.
Further, patent document 3 discloses the following method. Firstly, a frequency resource is allocated to a wireless station from which high priority data is to be transmitted. Also, a timing and a frame length in use for transmission of the high priority data are allocated thereto. When the high priority data is transmitted from an access point (AP) in accordance with the allocated timing and frame length, the wireless station checks channel availability by performing physical carrier sense before transmission, and transmits the high priority data only if channel availability is verified (i.e., the channel is idle).
However, in the case where the above-described communication system is a power line communication system, depending on the configuration of a device connected to a network, an amount of signal attenuation in one communication system may substantially exceed an amount of signal attenuation which interferes with the other communication systems due to the characteristics of a power line transmission path. That is, in the case where patent document 1 or patent document 2 is applied to the power line communication system and interference between the communication systems is performed by power control, there is a possibility that, depending on the device configuration, some devices may be unable to perform device communication in the system due to reduced signal intensity. Also, in the case of wireless communications, the similar phenomenon, that is, signal intensity is suddenly reduced despite the physical closeness, may occur due to attenuation of the signal intensity, which is caused by a shield.
The above-described conventional configuration does not allow one communication system to maintain the quality of communications performed therein by transmission power control while reducing interference with the other communication systems. Thus, throughput of each communication system is substantially reduced due to the interference between the communication systems, and it is difficult to perform communication bandwidth control.
Also, in the case where the control disclosed in patent document 3 is employed, the interference between the communication systems can be reduced by virtual carrier or physical carrier sense. However, it is impossible to assure quality of service (QoS) of a communication bandwidth.